Achieving savings through fines processing in the crushing circuit

An Alliance low water washer consists of an agitator and dewatering screen. Depending on the application, water is introduced through a series of spray bars.

As the quarrying industry experiments more with manufactured sand, so the focus falls on how to develop a process that maximises output while minimising energy and water consumption costs.

Demand for manufactured sand is increasing at a rapid pace as natural sand reserves deplete, environmental constraints tighten, and permit processes lag. As aggregate producers work to keep up with more stringent product specifications, the need for efficient crusher fines management is key to running a cost- effective operation.

Typical solutions for washing crusher fines include sand screws and cyclones, which require high water use. A standard sand screw needs about 12.5m3 per hour of water for every tonne of minus 75 microns (μm).

Even if an operation only produces 90 tonnes per hour (tph) and their -75μm is 15 per cent – that equals 15 tonnes of fines requiring 187m3 per hour to lift the silt over the screw.The water requirement for a 220 tph capacity cyclone dewatering plant will run between 450 and600m3 per hour).

The cost of using that much water, along with imposed environmental restraints, makes using a sand screw or cyclone plant prohibitive for many crushing operations.In addition, washing crusher fines with a sand screw or cyclone typically requires operators to transfer the material from their crushing circuit to a separate washing circuit.

The rehandling process calls for an additional wheel loader, driver, and generator — all added costs. The extra time it takes to haul this material slows down an operator’s ability to get product to market.

To provide a solution for this unaddressed problem, Superior Industries designed and built the Alliance low water washer. The unit allows operators to wash crusher dust within their crushing circuit, eliminating the cost of handling and hauling these fines to a separate washing site. Designed with a small footprint, the machinery is easily incorporated into an existing plant. Current customers have experienced 80 per cent less water consumption when washing straight off a dry feed in place of their previous solution.

The Alliance low water washer’s agitator mixes a dry feed with water, producing a thick slurry that discharges onto an attached dewatering screen. The agitator is equipped with a spray bar along its entire length for thorough cleaning and to keep material from building up along the sides. The blades are constructed of 25mm abrasion resistant cast iron.

Designed with a small footprint, the Alliance low water washer is easily incorporated into an existing plant.

Using a sand screw requires a large volume of water to lift the silt over the weirs of the screw. Alternatively, the low water washer pushes material down through the screen, a process that requires much less water.

After the agitator, material is discharged to an integrated dewatering screen, which reduces moisture content to as low as eight per cent. Its deep bed design removes moisture more effectively and retains the highest possible percentage of saleable product. Urethane sidewalls are lined with screen openings, creating a larger surface area for drainage.

Compared to cyclone plants or sand screws, the Alliance low water washer requires less power. A 220 tph cyclone plant typically runs at 93kW while a low water washer processing 181 tph runs at only 28kW. A 220 tph sand screw requires two 11 kW motors to operate.

Case Study: Producer achieves large throughput

Rilite Aggregates, based in Reno, Nevada, USA, faced a water management challenge. The company had been washing its concrete sand with a 1371mm twin screw that required a whopping 500m3 per hour of water.

“We had to fill a holding pond and utilise a large pump. With that, we could wash material for only three hours per day,” Michael Rudolph, a long-term crushing subcontractor who has worked with Rilite for nearly 20 years, said.

The water management situation reached a “crisis” mode when a specification change in utility bedding sand required the company to wash that product as well, lowering the minus 75μm from 15 per cent to seven per cent or less.

Rudolph said that Rilite needed an aggressive washing system that could run off available water. “With a drastic change in sand specifications, we were forced to add washing capacities, so we wanted a lower cost, lower maintenance solution that would limit water use. We have our own well but it only pumps 220 gallons per minute [50m3 per hour],” he said.

The local Superior Industries dealer, Kimball Equipment, referred Rudolph to John Bennington, Superior’s director of washing and classifying. Soon thereafter, Rudolph flew to a quarry in Michigan to see the prototype of the Alliance low water washer in action.

According to Bennington, the operation in Michigan processes 90 to 108 tph of feed through the low water washer, and generally runs from 45m3 to 57m3 per hour) The raw feed averages about 22 per cent of -75μm. At that rate, the conventional sand screw would require 250m3 per hour. “After washing, the operation is achieving under a six per cent -75μm, with a target of four per cent after further refinements,” he said.

At the Michigan site, Rudolph ran pre- and post-wash samples through the new washer and found that the unit would meet Rilite’s specification requirements. This led to the installation of the Alliance low water washer at Rilite Aggregates in February of 2017.

“I had the site ready to go, so it was a matter of unloading the truck, plus one day of hook-up, and we were in business washing bedding sand at 158 to 181 tph,” Rudolph said. “It handles that capacity in such a small footprint, allowing us to install the washer without making any modifications to the existing plant.”

He added that he placed a belt stacker in front of the washer that can swing to feed the washer or swing away to by-pass the washer when needed.

Rudolph said the low water washer averages about 68m3 per hour, which is fed from a large freshwater pond that acts as a surge system for the well.

“Versus the previous use of twin sand screws at 500m3 per hour, I now vary my flow rate to the low water washer between 45m3 and 90m3 per hour. This has dramatically lowered our washing costs. We can conserve water, and manage its use without jeopardising our need for water trucks for dust control,” he stressed.

Importantly, Rudolph pointed to the moisture content of the material after washing. “It’s between 11 and 13 per cent. With that, we can sell the product within one day. With our previous set-up, we had 22 per cent moisture content after washing and had to let the material sit and dry for about a week,” he said, summing up the biggest advantage of “low water use and large throughput”.

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